This invention relates to an ignition apparatus for internal combustion engines which is of the capacitor discharge type using a battery as a DC power supply.
There is known an ignition apparatus of this type in which an oscillation-type DC-DC converter is used for boosting the output voltage of the battery and charging a capacitor (for example, U.S. Pat. No. 3,599,616 specification), and there is known another one in which a current from the battery is flowed in the primary coil of a boosting transformer in synchronism with an ignition signal and when reaching a constant value, the current is cut off to cause in the secondary coil thereof a high voltage, by which the capacitor is charged (for example, Japanese Utility Model Publication Gazette No. 57319/1977).
In the former one of the conventional ignition apparatus, however, the oscillation transformer of the oscillation-type DC-DC converter needs a ternary coil for self-oscillation in addition to the primary and secondary coils and thus it makes the apparatus complicated in construction. Also, since at high speed of engine, the capacitor must be charged enough, the oscillation frequency of the oscillation transformer is inevitably increased, causing radio-wave noise which aversely affects other electronic equipment. Since the number of times that the capacitor is charged per cycle of ignition is changed as the revolution rate of engine increases or decreases, the charged voltage across the capacitor changes with the change of the revolution rate of engine and as a result the ignition energy changes with the change of the revolution of engine. Moreover, since the oscillation circuit is required, the apparatus becomes large-sized and expensive.
In the latter one, since the primary current of the boosting transformer is controlled to start flowing by the ignition signal, a high voltage is only once induced per cycle of ignition, in the secondary coil of the boosting transformer and the capacitor is charged by the voltage induced once per cycle of ignition. Therefore, in order to obtain energy necessary to ignite by this single charging operation per cycle of ignition, the primary current of the boosting transformer and the number of turns of the primary coil must be increased to increase the electromagnetic energy (1/2.multidot.L.multidot.I.sup.2) stored in the primary coil. Moreover, since the core of the boosting transformer must have a large cross-sectional area so that the magnetic flux produced in accordance with the ampareturn of the primary coil cannot saturate, the boosting transformer becomes large-sized. Also, since a large-capacity transistor is required for cutting off the primary current, the apparatus becomes large-sized and expensive. Furthermore, in the prior art, no consideration is made for the over-rotation prevention.